1. Field of the Invention
The present invention relates to a fuel injection valve used for an internal combustion engine. Particularly, it relates to an air assist type fuel injection valve.
2. Discussion of Background
FIGS. 6 through 8 are diagrams showing a conventional air assist type fuel injection valve disclosed in, for instance, Japanese Unexamined Patent Publication No. 264764/1991 wherein FIG. 6 is a longitudinal sectional view partly omitted showing an air injection system; FIG. 7 is a side view showing a fuel injection valve in FIG. 6, and FIG. 8 is a longitudinal cross-sectional view of an adapter main body for mixing air with fuel in the fuel injection valve shown in FIG. 7. In the Figures, reference numeral 1 designates an injector main body in an electromagnetic type fuel injection valve, numeral 2 designates an adapter main body attached to the bottom end portion 1a of the injector main body 1 to mix air with fuel, numeral 3 designates an air passage for air injection, and numeral 4 designates a fuel injection opening formed at the bottom end of the injector main body 1.
In operation, when an electric current is supplied to the injector main body 1, a needle valve is opened, and fuel is fed through the fuel injection opening 4 at the bottom end of the injector main body 1. At the same time, a predetermined amount of air is supplied to an air-fuel mixing portion 5 through the air intake passage 3 of the adapter main body 2 which is fixed to the bottom end portion la of the injector main body 1. Then, the fuel fed through the fuel injection opening 4 collides with air and is mixed with it in the air-fuel mixing portion 5, and the fuel becomes fine particles. The fuel is sprayed outside in the form of a mist. The shape of the mist is determined by the shape of the air-fuel mixing portion 5 formed in the adapter main body 2.
In the conventional air assist type fuel injection valve, since the shape of the air-fuel mixing portion 5 formed in the adapter main body 2 is cylindrical, there was a problem that atomized fuel deposits on the inner wall 6 of the air-fuel mixing portion 5, resulting in a liquid dripping phenomenon.
In order to prevent the disadvantage of the liquid dripping phenomenon, there was proposed that the ratio of the length L of the air-fuel mixing portion 5 to its inner diameter D was 1 or less. However, such technique increased a spray angle (30.degree. or more) whereby the optimum shape of spray could not be obtained. Further, there was proposed such a technique that the air-fuel mixing portion 5 was formed to have a tapered shape as shown in FIG. 9. However, this technique had a problem that the rate of pressure change was increased at the fuel injection opening 5 between the time of injecting air and the time of non-injecting, although the occurrence of liquid dripping could be suppressed, so that the flow rate of fuel between the air-injection time and the non-air-injection time was largely changed (for instance, 5% or more) .